The present invention relates to a vibrator employing a piezoelectric ceramic and, more particularly, to an assembly of the piezoelectric vibrator of the energy trapped type producing vibration in thickness mode.
Conventionally, there have been proposed various types of the above described oscillator which is suitable for use in filters, resonators, oscillators, traps and discriminators. A typical piezoelectric vibrator is shown in FIGS. 1a and 1b and includes a thin plate 1 made of piezoelectric ceramic material, a pair of electrodes 2a and 2b deposited on opposite flat surfaces of the plate 1 in face-to-face relation to each other at approximately center portion of the plate 1, an intervening electrode 3a extending from the electrode 2a towards a terminal electrode 4a located at a corner of the plate 1, and an intervening electrode 3b extending from the electrode 2b towards a terminal electrode 4b located at another corner of the plate 1. A pair of terminal legs 5a and 5b extend from the terminal electrodes 4a and 4b, respectively, for the external electrical connection to the electrodes 2a and 2b. When alternating electrical potential is applied between the electrodes 2a and 2b, an alternating electric field is generated that causes localized vibration in the plate 1 between the electrodes 2a and 2b. The vibration is propagated radially towards the edges of the plate 1 in a form of thickness shear or thickness expansion mode to give rise an oscillation in the plate 1. Since the oscillation is mainly carried out in the center vibrating portion of the plate 1, the vibrating energy can be trapped in the center portion of the plate 1 as understood from the theory of energy trap. Therefore, even if an inactive portion or a portion which scarcely vibrates is held tightly, the vibration occurring at the center active portion can be activated without any disturbance. Accordingly, the conventional vibrator is further provided with a coating 6 which completely surrounds the plate 1 with cavities 7a and 7b formed around the electrodes 2a and 2b. The coating 6 serves as a damper to hold the non-vibrating portion of the plate 1, and also to suppress the unwanted waves.
Another prior art vibrator is shown in FIGS. 2a and 2b having a first pair of separate electrodes 8a and 8b and a second pair of separate electrodes 9a and 9b deposited on one flat surface of the plate 1. Ground, or counter, electrodes 10 and 11 are deposited on the other flat surface of the plate 1 at places corresponding the first and second separate electrodes 8 and 9, respectively. The separate electrodes 8a and 9a are connected with each other and the separate electrodes 8b and 9b are connected to terminal legs 5a and 5b, as shown. The ground electrodes 10 and 11 are connected to a terminal leg 5c. This prior art vibrator, which is generally called a double unit type in contrast with a single unit type shown in FIGS. 1a and 1b, is also provided with coating 6.
The above described prior art vibrators are disadvantageous in that the plate 1 must be considerably large to enable the deposition of terminal electrodes 4a and 4b and also to apply the coating 6 and that the terminal legs are required for the external electrical connection. Furthermore, the step necessary to provide the coating 6 increases the manufacturing cost.
Accordingly, a primary object of the present invention is to provide a piezoelectric vibrator of the above described type which is compact in size and simple in construction.
It is another object of the present invention to provide a piezoelectric vibrator of the above described type which can be readily manufactured at low cost.